A typical nuclear steam generator comprises a vertically-oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube-sheet for supporting the tubes at the ends opposite the U-like curvature, a dividing plate that cooperates with the tube sheet forming a primary fluid inlet plenum at the one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle. A primary fluid inlet nozzle is in fluid communication with the primary fluid inlet plenum, and a primary fluid outlet nozzle is in fluid communication with the primary fluid plenum. This configuration is described for example by U.S. Pat. Nos. 4,079,701; 4,723,076; 4,899,697 and 4,921,662 (Hickman et al.; Lahoda et al.; Franklin et al. and Franklin et al.; respectively).
Since the primary fluid contains radioactive particles and is isolated from feedwater only by the U-tube walls, which may be constructed of Inconel®, the U-tube walls form part of the primary boundary for isolating these radioactive particles. It is, therefore, important that the U-tubes be maintained defect-free so that no leaks/breaks will occur in the U-tubes.
It has been found that there are at least two causes of potential leaks in the U-tube walls. High caustic levels found in the vicinity of the cracks in tube specimens taken from operating steam generators and the similarity of these cracks to failures produced by caustic under controlled laboratory conditions, have identified high caustic levels as the possible cause of the intergranular corrosion, and thus the possible cause of the tube cracking.
The other cause of tube leaks is thought to be tube thinning. Eddy current tests of the tubes have indicated that the thinning occurs on tubes near the tube sheet at levels corresponding to the levels of sludge that has accumulated on the tube sheet. The sludge is mainly iron oxide particulates and copper compounds along with traces of other minerals that have settled out of the feedwater onto the tube sheet, and into the annulus between the tube sheet and the tubes. The level of sludge accumulation may be inferred by eddy current testing with a low frequency signal that is sensitive to the magnetite in the sludge. The correlation between sludge levels and the tube wall thinning location strongly suggests that the sludge deposits provide a site for concentration of phosphate solution or other corrosive agents at the tube wall that results in tube thinning.
Additionally each of the U-shaped heat exchanger tubes has a “hot leg” U-bend at its top and both “hot and cold legs” at the bottom end of each heat exchanger. Usually the bottom hot and cold legs are sludge treated/suctioned separately.
A number of patents have previously described moveable, high pressure, single head, sludge lance-suction methods of removing top tube sheet sludge including, for example, the patents previously set out, as well as U.S. Pat. Nos. 4,276,856; 4,572,284; 4,676,201; 4,774,975; 4,971,140; 5,036,871; 5,069,172; 5,615,734; 5,813,370; 6,513,462; and 7,967,918 (Dent et al.; Katscher et al.; Lahoda et al.; Ayres et al.; Stoss; Ruggieri et al.; Shirey et al.; Hyp; Owen et al.; Shiraishi et al.; and Collin et al., respectively). These sludge removal methods are utilized after an initial chemical cleaning which reduces the hard (tenaciously adhering) sludge on the tube sheet, especially in a “kidney” shaped high accumulation region in the hot leg zone, to a generally particulate film.
In most nuclear steam generators in service today, there are usually 6 inch (15.2 cm.) diameter hand holes in the shell of the steam generator near and above the tube sheet that has an associated hole in the wrapper providing access to the tube sheet for removal of the sludge deposits on the tube sheet.
In all the above apparatus, the single head used must stop at a central rod in the tube sheet, so that the central row of tubes across the tube lane and the hot and cold leg is difficult to clean, and that central row of tubes also crosses the middle of the “kidney” region of the hot leg. It is essential to remove sludge from 100% of the tubes and tube sheet surface. Leaving 5% or 10% of the sludge removal in a marginal state jeopardizes the entire sludge removal process, since it only takes a single leaking tube for potential contamination by radioactive particles from the primary fluid which is under high pressure and at about 650° C. Thus there is a need for a method that can clean that central row effectively, and a main object of this invention is to provide such a method and apparatus.